The Syrian Observatory for Human Rights said that the Syrian forces "opened a loophole" that led them into the base.

Rebel fighters stormed the base last November in a drive to relieve pressure on Eastern Ghouta's towns and villages.

The base had been used to strike at the densely populated Eastern Ghouta rebel enclave to attempt to force it into submission. More than 300,000 people there have lived under siege by army troops since 2013.

Pressing Idlib offensive

Meanwhile, in Idlib province, Syria's army and its allies pressed farther into the country's biggest remaining rebel stronghold on Sunday, capturing a town and several villages as they approached a military airport, a pro-Damascus media unit and a war monitor reported.

Forces loyal to President Bashar al-Assad have stepped up the offensive in the southern province of Idlib in recent days, advancing towards the Abu al-Duhur airport, which rebels captured from the president's troops in September 2015.

The Syrian army and its allies "have gained control over the town of Sinjar," 14km from Abu al-Duhur, and three villages to the west, the media unit run by Assad ally Hezbollah reported.

Supported by Iran-backed militias and Russian air power, Assad's forces have since late October taken back rebel-held territory in Idlib and the northeastern province of Hama.

The Syrian Observatory for Human Rights said on Sunday the army had taken more than 95 villages in Hama and Idlib since 22 October, including about 60 in Idlib alone during the past 14 days.

Displaced Syrians flee fighting in Idlib province's southeastern town of Sinjar, taking road through rebel-held area near city of Saraqib on Sunday (AFP)

"Battles have shifted now to the northwest of Sinjar after the Syrian army and its allies controlled the town," the Britain-based monitor reported.

The main rebel force in Idlib is Tahrir al-Sham, spearheaded by the former al-Qaeda affiliate in Syria previously called Nusra Front.

The Syrian army lost Idlib, which borders Turkey, to rebels when the provincial capital fell in 2015. It became the only province fully under opposition control.

The largest population gathering is in Eastern Idlib, the monitor said, expressing fears of further intensive civilian displacement in the coming few days amid ongoing fighting.

Tens of thousands of rebels and civilians took refuge in Idlib after abandoning their homes in other parts of western Syria that the government and its foreign allies have retaken.

The fighting and air strikes have forced more than 60,000 people to leave their homes since 1 November, according to the UN Office for the Coordination of Humanitarian Affairs.

The United Nations said the civilians newly displaced by the fighting in Idlib were in a "dire" situation.